The present invention relates to hot-worked permanent magnets consisting substantially of rare earth elements, transition metals and boron and provided with magnetic anisotropy by hot working, and more particularly to hot-worked magnets having improved crystal grain orientation and thus having good magnetic properties. The present inventions especially relates to a method of producing such hot-worked magnets without cracking by adding proper amounts of additives as graphite powder and glass material having a low melting point to improve workability.
Permanent magnets consisting essentially of rare earth elements, transition metals and boron (hereinafter referred to as "R-T-B permanent magnets") have been receiving much attention as inexpensive permanent magnets having excellent magnetic properties. This is because intermetallic compounds expressed by R.sub.2 T.sub.14 B having a tetragonal crystal structure have excellent magnetic properties. Nd.sub.2 Fe.sub.14 B, in which Nd is employed as R, has lattice parameters of a.sub.0 =0.878 nm and c.sub.0 =1.219 nm.
The R-T-B permanent magnets are usually classified into two groups: sintered magnets and rapidly quenched magnets. Whichever production method is utilized, it is necessary to form them to desired shapes. In this sense, they should have good workability. In order to improve the workability of the magnets, the addition of lubricating agents has conventionally been conducted. The lubricants are classified into external lubricants which are applied to die surfaces or surfaces of magnet products to be formed to reduce friction between the die surfaces and the magnet products being formed, and internal lubricants which are in the form of powder, liquid, solid, etc. and are added to the magnet products to be formed to reduce friction between the powder particles.
European Patent Laid-Open No. EP 0,133,758 discloses the coating of a die surface with graphite as an external lubricant for hot die-upsetting, to improve the workability of magnets in the hot-working process, thereby obtaining hot-worked magnets free from cracks. The effects of graphite on the inner lubricantion of the magnets are not referred to. U.S. Pat. No. 4,780,226 discloses a method of producing a hot-worked magnet wherein there is used a complex additive of graphite and glass material as an external lubricant for hot die-upsetting, to improve the workability of magnets in the hot-working process. In the method, a glass powder material having a melting point which is lower than the hot-working temperature, or a mixture of glass powder and graphite powder is sprayed on the surfaces of punches and dies to form a green body of magnet material.
In the case of sintered magnets, stearic acid is widely used as an internal lubricant (Japanese Patent Laid-Open No. 61-34101). Stearic acid is a saturated aliphatic acid having the formula: CH.sub.3 (CH.sub.2).sub.16 COOH. It is also known to suppress the growth of crystal grains and simultaneously increase the density of the resulting magnet in the sintering step by adding carbon powder or a powder of carbide-forming components such as Ti, Zr, Hf, etc. to form metal carbides (Japanese Patent Laid-Open No. 63-98105).
However, if sintered magnets are to be provided with magnetic anisotropy, a pressing step in a magnetic field must be conducted, limiting the shapes of magnets to be formed. In view of this fact, much attention has been paid to rapidly quenched magnets which do not need to be pressed in a magnetic field, particularly permanet magnets obtianed by pulverizing thin ribbons or flakes produced from melts of R-T-B alloys by a rapid quenching method, hot-pressing them (high-temperature treatment) and then subjecting them to plastic working at high temperature to provide them with magnetic anisotropy, which will be called "hot-worked magnets" hereinafter (European Patent Laid-Open No. EP 0,133,758). The individual thin ribbons or flakes produced by such a rapid quenching method usually contain innumerable fine crystal grains. Even though the thin ribbons or flakes produced by rapid quenching are in various planar shapes of 30 .mu.m in thickness and 500 .mu.m or less in length, the crystal grains contained therein are as fine as 0.02-1.0 .mu.m as an average grain size, which is smaller than the average grain size of 1-90 .mu. m in the case of sintered magnets (see, for instance, European Patent Laid-Open No. EP 0,126,179). The average grain size of the rapidly quenched magnets is close to 0.3 .mu.m, the critical size of a single domain of the R-T-B magnet, which means that it provides essentially excellent magnetic properties.
In the case of hot working rapidly quenched magnetic materials, it is important that there is a close relationship between the direction of plastic flow and magnetic orientation perpendicular to the direction of the plastic flow. Further, it is necessary to cause the plastic flow uniformly in the entire magnet to be worked, in order to improve the orientation of the crystal grains which strongly influence the magnetic properties. Incidentally, a nonuniform deformation may cause bulging of the magnets in the plastic working process, which in turn produces large and/or many cracks in the peripheral portions of the magnets. This is a serious problem when hot-worked magnets are to be formed into the shape of final products. Most of the force applied in hot-working is used for plastic deformation, but part of the force is exhausted by friction between the particles. This may be a partial cause of the above bulging phenomenon.
Various types of internal lubricants for hot-worked magnets are known in the art. EP 0,195,219 discloses a rapidly quenched hot-worked permanent magnet of the R-T-B type in which each particle of the powder material used for the preform may be coated with an inorganic or organic lubricant. Examples of suitable lubricants given are graphite and molybdenum disulfide. Japanese Laid-Open No. 60-184,602 discloses the use of polyethylene glycol monolaurate to increase the formability of sintered magnets, and U.S. patent application Ser. No. 07/327,631 (commonly assigned) discloses the use of various organic compounds as internal lubricants to provide carbon and oxygen in the grain boundaries after the hot-working step.
In the above-mentioned conventional techniques, external lubricants such as graphite and/or glass applied to the die surface for die lubrication to reduce friction between a work body and surfaces of tools (dies and punches) only partly, if at all, attaches to the thin ribbons or flakes produced by a rapid quenching method, which are 30 .mu.m or so in thickness and 500 .mu.m or less in length, much less to the innumerable fine crystal grains inside the thin flakes. Hence, external lubricants do not play a role as an inner lubricant to reduce occurance of cracks in a magnet produced by hot-working.
Incidentally, in the case of adding carbon powder or powder of carbide-forming components such as Ti, Zr, Hf, etc. to sintered magnets, it is expected that such powder is relatively easily dispersed in magnet powder by appropriately selecting a powder shape and a mixing method. The same is true of stearate. This is because in the case of sintered magnets, magnetic powder particles produced by pulverizing alloy ingots are in a shape close to spherical. However, unlike the sintered magnets produced by powder metallurgy method in which compacting is conducted at room temperature, hot-working such as by die-upsetting, is usually conducted at as high a temperature as 600.degree.-850.degree. C. Accordingly, lubricants dispersed among thin flakes show essentially different behavior, and this has not yet been appreciated.
In addition, the conventional techniques in which an external lubricant is applied to a die surface do not show effects peculiar to the hot-working of the magnets, but they simply show effects of lubricants which slightly decrease the friction between the die surface and materials being worked. In fact, there has been no report so far with respect to the improvement of workability without significant cracking and the improvement of uniform orientation, in the field of hot-working rapidly quenched magnet ribbons or flakes.